Archive for March 28th, 2011

Putting a geared stepper motor on the MK5 filament drive produced greatly improved print quality, which meant I could finally print ScribbleJ’s version of the classic Wade Filament Tensioner and expect that it’d come out right. It’s a rather large lump of plastic that printed quite nicely on an aluminum plate.

Wade-ScribbleJ Filament Tensioner on plate

The whole thing looks like this when it’s all assembled and adjusted:

Complete Geared Stepper Extruder

[Update: The motor comes directly from the usual eBay supplier. You won’t find another one like it, but this is directly from the label…

38 mm case

Minebea-Matsushita 17PM-K150-P1V

No. T6824-02

]

You can see the two thermal switches near the bottom of the picture. The 100 °C switch is inside the acrylic frame on the Thermal Riser, the 40 °C switch is just taped to the filament tensioner’s base. The former is OK, the latter isn’t as secure as it should be. FWIW, Riser temperatures run around 70 °C during normal extrusion, albeit in a chilly Basement Laboratory without covers on the TOM’s windows.

A better view of the shaft bearings and filament position:

Filament tensioner - overview

The four long screws are 1.5 inch 4-40 from my heap, although 2 inch screws would give more room for adjustment. Some folks mount the screws the other way around, with the nuts pressing on the springs and little knobs on the nuts. I gave up on the washers to get a bit more adjustment range.

The springs came from my Big Box o’ Little Springs, sporting absolutely no pedigree at all. They’re not quite completely compressed, so there’s a bit of push left in them to handle filament diameter variations (which is the whole point of this exercise). I added four nuts (between the shaft bearing plate and the idler block) to keep the idler block from resting against the drive gear when there’s no filament in place: inserting a new filament became much easier.

Somewhat to my surprise, the entire filament drive gear assembly is free-floating and self-aligning within the housing:

Filament drive gear detail

I enabled the option to put a 5 mm diameter cleanout hole in the bearing housing, which turned out to be absolutely essential for monitoring the location of the drive gear inside all the machinery. You can barely see the hole in the first picture, on the left side of the curved section.

A floating shaft means the 7-tooth motor drive gear’s position must line up with wherever the 51-tooth filament drive gear happens to be. There’s not much room to adjust the motor gear, but a few iterations sorted out the proper number and placement of all the filament drive shaft washers, nuts, and bearings.

Filament drive gear - shaft spacing

You (well, I) really really must put a flat on the shaft and use full-strength Loctite to hold the setscrew in place. I used an all-thread M6x40 bolt because that’s what I had on hand, but a partially threaded M6x50 bolt would provide better support for the bearings, more clearance for the spacers, and look a lot better; it’d require a custom-turned bushing instead of the nut against the big gear, though.

Flatted filament drive shaft

I initially used low-strength Loctite. Word: a loose drive gear setscrew can convince you that Skeinforge’s Reversal plugin isn’t working after you make many changes with worsening results. Those fast reversals loosen the setscrew in short order.

The diameters of the 7- and 51-tooth herringbone gears determine the center-to-center distance between the motor shaft and the extruder shaft. Alas, two of the motor mounting bolts wind up directly behind the larger gear. I marked the gear adjacent to the bolt heads and drilled a hole that just barely admits the hex wrench:

Stepper Extruder - motor mount access hole

If you knew where that hole was supposed to be, you could print it right into the gear, but I haven’t a clue as to how you might algorithmically determine the precise location on the as-printed gears.

The modified OpenSCAD source produces two recesses for the bolt head and nut, but I just applied an end mill to the head side of the finished idler block. There’s no room for the bolt head between the block and the motor mounting plate.

Idler housing with recessed bolt

Of course, I modified the OpenSCAD code along the way:

Changing the bearing size moved the base: use front_bearing_r in the routine that punches the holes

Add a complete outer surface on the idler block; I thought I might want a flat metal plate to distribute the stress.

Add bolt head / nut recesses for idler block pulley shaft

Include base_wall_h in the calculation for idler_max_h

Tweaked spacing to get idler bolt heads out of the walls

Extend motor wall rightward to cover all of the base plate

Adjust base hole positioning: -10 / +4.0, not -10 / +3.5

But not all instances of 3.5 must change, I think

Filament offset may need further tweakage

Other miscellaneous tweaks

Not all of those changes made it to the printed object shown here; if I ever print another one, they’ll be included. Use at your own risk!

One a day, every day…

Redshifting

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